The reduction of emissions from vehicle exhaust systems is a well known problem. As the number of vehicles having an internal combustion engine continues to increase, the problem is becoming more severe and despite the introduction of catalytic converter exhaust systems, the exhaust emissions from vehicles fitted with such systems are still relatively high.
In particular exhaust emissions are relatively high during the initial warm-up phase of an internal combustion engine after starting, especially with regard to the emissions of carbon monoxide, oxides of nitrogen and hydrocarbons. This is largely due to the fact that the catalytic converter has not reached its so-called "light-off" temperature, at which the catalyst constituted by the noble metals (typically platinum, rhodium and palladium present in the washcoat matrix of the catalytic converter brick) causes the required catalytic reactions to take place. The light-off temperature can be defined as the temperature at which the catalytic converter reaches 50% conversion. Modern catalyst systems-start operating at temperatures of around 200.degree. C. to 300.degree. C.
In order to reduce the quantity of harmful emissions during said initial warm-up phase, a plurality of different solutions has been proposed, many of these solutions being based on shortening the time taken to reach the light-off temperature by raising the temperature in the catalyst as fast as possible.
An example of one of these systems is one making use of an exhaust afterburner (see e.g. EP-A-0 422 432), comprising a combustion chamber in which exhaust products present in the exhaust gases are ignited so as to raise the temperature of the gases passing through the catalytic converter and thereby, in turn, raise the temperature of the catalytic converter itself, so that light-off is quickly reached.
The required ignition of the gases can be performed with the aid of an ignition device which is placed either upstream of the catalytic converter or between two stages (i.e. beteween two "bricks") of the catalytic converter.
Such devices have however suffered from the problem that the quantity of unburned hydrogen and oxygen is often insufficient to produce a rapid and reliable heating of the catalytic converter.
In order to overcome these drawbacks, WO-A-92/22734 and WO-A-93/07365 each discloses a system whereby the hydrogen and oxygen mixture which reaches the ignition zone in the afterburner combustion chamber is readily ignitible by the ignition device immediately after cranking of the engine. This is achieved by ensuring that the concentration of hydrogen and oxygen remains within known flammability limits. In order to obtain the required concentrations, the fuel/air mixture is enriched significantly so as to obtain additional hydrogen, whilst additional oxygen is added by means of a supplementary air pump.
Although an improvement is obtained hereby, a severe drawback of the system is that an ignition device is required in the afterburner. Such an ignition device constitutes an extra component which is prone to failure. Moreover, from the consumer point of view, this is undesirable due to the resultant extra cost involved with the more expensive exhaust system and the ensuing costs of servicing and/or replacing worn-out or faulty afterburner ignition devices.
A further disadvantage presents itself in that exhaust oxidation or heat energy generated by the combustion in the engine is to a great extent lost during transport thereof to the catalytic converter.
Another disadvantage of prior art systems which supply extra air to the exhaust system is that additional air supply equipment is required. This extra equipment is costly to provide and may thus affect the final vehicle price significantly. Moreover, the use of such additional equipment, especially when added to existing engines and existing vehicles, requires space for fitting of the supply equipment around the engine and bodywork parts. Such fitting can be very difficult to accommodate and significant re-design of existing vehicles, their engines or their exhaust systems may be required. This will also add to cost of course. Thus, in a preferred embodiment of our invention, means are provided for obviating the need for such hardware.
An additional disadvantage is that such auxiliary equipment will increase the weight of the vehicle.